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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Winter 1995 systems. Lower soil temperatures mean less N mineralization from soil organic matter early in the season. This is why it is essential to provide the crop with supplemental N to assure robust early growth. This can be accomplished by applying 20 to 40 lbs/A of N in starter fertilizer or by broadcasting 30 to 60 lbs/A of N near time of planting. Stratification. Non-inversion tillage systems result in stratification of nutrients within the soil profile. For soils with depleted nutrients due to stratification, the need for starter fertilizer is increased. Also, cooler, wetter conditions, which usually result in slower release of nutrients from the soil, may suggest a need for starter fertilizer. Immobile nutrients such as phosphorus and potassium usually decrease with soil depth. Available P and K will usually be concentrated in the upper 2 to 4 inches of the soil. In a Wisconsin corn study, a starter application of 80 lbs/A of phosphate and 80 lbs/A of potassium produced more than double the yield increase (35 bu/A vs. 14 bu/A) in a reduced-till field versus conventional-till (Figure 2), again demonstrating how starters benefit crops in reduced-till situations where soil temperatures are lowered and nutrients are tied up by residue. Root growth. The direction of root growth for a young seedling is usually downward. Under heavy residue systems, roots may concentrate near the soil surface due to the cool temperate and higher moisture conditions. This permits good nutrient uptake from the surface soil later in the season, but does not supply the seedling with needed nutrients. If residue levels are low, the need to incorporate nutrients deeper in the soil profile will be magnified. Plants will not concentrate roots at the soil surface if the soil dries out and temperatures are high, as would be anticipated when soil surfaces have little or no mulch. Table 1 shows the payoff of deep fertility, one of the common denominators in producing high yields. Note the extremely high levels of phosphate and potassium in the top 12 inches, which create an environment of greater soil volume for proliferation of roots as they search for nutrients and water. Volatilization. One consequence of applying urea-based fertilizers to reduced- till soils is an increased probability of surface volatilization. The urease enzyme in the soil and plant residue breaks down urea to form ammonia. At the surface of the soil or on the residue there often is not enough moisture to trap this ammonia, resulting in significant N losses. Conditions that control the rate of conversion are temperature, pH and the presence of the urease enzyme. There are several ways to minimize conversion: 1. Apply N when soil temperatures are lower 2. Lime soil when pH rises, applying it as far ahead of planting time as possible 3. Reduce contact of fertilizer with the urease enzyme by concentrating N in bands (i.e., dribbling 28% UAN ) 4. Apply N where there is adequate moisture to absorb the ammonia 5. Place the N below the soil's surface. Figure 3 shows how knifing (versus broadcast) produces superior wheat yields. Organic matter. Reduced-till results in the accumulation of organic matter near the surface of the soil, producing higher available carbon for microbial growth. The higher residues also reduce surface evaporation of water. These two conditions may result in higher potential for denitrification losses. Injection of N below this layer should reduce denitrification of N. Nitrification inhibitors can also help. A third alternative is to split apply N, applying most of the N at side-dress time when surface soil is seldom wet for a long period and plant uptake rapid. Dr. Johnson is the professor of Soil Fertility at Ohio State University. ! Pioneer 3183 Funks G4507 Hybrid Plant population/A:25,000 Plant population/A:37,000 Yield - bu/A 220 210 200 190 180 170 160 150 140 Figure 1. Effect of hybrid selection and plant population on cor n yield, Olsen, Colorado State University. Yield - bu/A No Starter Starter 120 110 100 90 80 70 60 50 Conventional Till Reduced Till Figure 2. Effect of starter fertilizer on cor n yield in reduced-till vs conventional-till system, Wisconsin . Check Broadcast Knifed Method of Application 70 60 50 40 30 20 10 Yield - bu/A Figure 3. Comparative effectiveness of banding vs. knifing, (75-40-0 treatments), Kansas.
Fluid Journal 1996-1998